You should feel 4WD when making a tight turn, High or Low, the front wheels will feel different, you may feel a lurching.
No, in 4WD High or Low, if everything is working right, you have all 4 wheels working full time.
'99 does have the infamous PVH(Pulse Vacuum Hubs), the previous Ford "auto hubs" were getting a bad reputation..............so Ford decided to make it worse, the 1998-2000 Ranger 4x4's came with PVH.
2001 saw "live axles", lol, no locking/unlocking, just transfer case shifting in and out of 4WD.
It may be hard to tell if 4WD is working, but it is isn't I would read here just to check out the most likely problem, PVH: Ford Ranger How To: Troubleshoot/Fix Your PVH Hubs (1998-2000)